Major Concepts of Physics PHY102
Lecture #11
Reflection. Geometrical optics
Monday, February 29th
Spring 2016
Prof. Martin Forstner
lmovilea@syr.edu
Room 211, Physics Bldg., 443-8078
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Lecture objectives
1. The ray approximation (essential for
Geometrical Optics)
2. Geometrical optics
3. Reflection
4. Applications
5. Announcements
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Alexander Graham Bell – 1870s
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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The light flash of a firefly
Oxygen reacts with luciferin.
The reaction is catalyzed by the enzyme luciferase.
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Concentric circular ripples
Circular waves
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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The Ray approximation – Essential for
Geometrical Optics
The direction of travel of the electromagnetic wave (propagation
direction) is the ray direction.
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Conceptual example
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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The Ray Approximation
The RA says:
If light travels in a uniform medium, its rays are
straight lines.
The wave-like properties can be ignored.
Effectively, it assumes that the wavelength λ
is zero.
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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When the RA is Invalid
The opposite case is
 >> d.
The result then is that the opening behaves like a point
source of light.
The light spreads out from the opening.
The RA is not valid.
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Validity of the RA
Let d be the dimensions of any object or
opening, or obstacle, encountered.
If  << d,
then the RA is valid.
Ex.: Red light ( = 0.7 microns=700 nm) is
incident on a barrier. It has an opening of
diameter d = 1 cm. Then,
 << d.
So, the RA is valid.
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Ray diagrams
ray approximation
indicate the propagation of light
by arrows pointing in the direction
of propagation and perpendicular
to the wave fronts
valid approximation if
 << the size of any hole or objects
doesn’t take into account
interference or diffraction
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Geometrical Optics
This is the study of the paths that the light rays take,
when the RA is valid.
In contrast, physical optics studies phenomena that
develop from the fact that light is a wave.
Geometrical Optics is summarized by two laws:
The Laws of Reflection and Refraction
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Reflection
When light is incident on a boundary with a new material,
there are three possibilities:
It is absorbed.
1. For an ideal black body, this is the only process that
takes place.
2. It is transmitted into the new material.
We consider this later.
3. It is reflected.
In this case, part of the light energy remains in the original
material. Generally, all three processes occur.
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Reflection
We see all objects via light reflected from that object,
unless the object is itself a source of light.
Two Types of Reflection
1) Mirror (smooth surface)
2) Diffuse (rough surface)
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Big deal: the quality of surface!
Specular reflection
Diffuse reflection
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Criteria
When the surface of reflection is smooth, 1) takes
place.
If it is rough, 2) takes place.
What determines smoothness?
A smooth surface has surface variations that are much
less than .
For a rough surface, the variations are larger than .
Because  is small for light, most surfaces are rough.
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Laws for Diffuse and Mirror
Reflection
In diffuse reflection, the light is reflected in all directions.
In mirror reflection, a simple law holds:
Let i be the angle of incidence.
Always, angles are measured from the direction of the
surface normal.
The normal is an imaginary line, perpendicular to the surface,
at the point of contact of the ray.
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Statement for the Law of Reflection
Statement of the Law of Reflection
Let r be the direction of the reflected ray. Then,
inc = refl .
The angle of reflection is equal to the angle of incidence.
Plane of the Reflected Ray
The reflected ray is in the same plane as the incident ray
and the normal.
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Reflection
light waves can be reflected at
a surface between two media
Law of Reflection
inc = refl
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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Reflection
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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The Virtual Image
Flat mirror forms virtual image
Virtual image is similar to the object
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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The Virtual Image
Draw these reflected rays. They reach the eye of an observer.
The rays appear to come from a point behind the mirror.
This point is the image-point.
All the rays appear to diverge from the image-point.
The rays of light do not actually come from the image-point.
So, we say that the image is a virtual image.
From simple geometry, it is easy to prove the following:
The object-size and the image-size are the same. The distances to the
mirror of the image and the object are the same.
Major Concepts of Physics PHY102 – Lecture #11
2016Syracuse University
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Great discoveries are based on
simple principles!
Fig. 25.6 in the textbook
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Announcements
1. Reading:
Chapter 23rd, Sections 23.1, 23.2, pp. 860-865.
Conceptual examples 23.1
2. Homework. Homework #4 is due on this week’s lab.
3. This week’s workshop: Reflection and Mirrors
Major Concepts of Physics PHY 102 – Lecture #11 2016Syracuse University
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